Anti-pollution method

ABSTRACT

Removal of fluoride from a stream of gas contaminated therewith by sorption on alumina moving in the same direction and in contact therewith for not exceeding 20 seconds.

United States Patent 1 [111 3,760,565 Fish 1 Sept. 25, 1973 [54]ANTI-POLLUTION METHOD R27,383 5/1972 Johnson 204/67 204 [751 Inventor:William Fish, Des 32333333 131323 332321. 2313; [73] Assignee: AluminumCompany of America, Pittsburgh, Pa. Primary ExaminerJohn H. Mack Fllediy 1971 Assistant ExaminerD. R. Valentine [21 Appl 1 3 715 Att0rneyAbramW. Hatcher [52] US. Cl 55/71, 55/79, 204/67,

423/240, 423/241, 423/495 [57] ABSTRACT [51] Int. Cl. 301d 53/06, C22d3/12, COlb 7/00 Field 0f Search Removal of fluoride from a stream of gascontaminated 204/247; 42 3/ 79 therewith by sorption on alumina movingin the same direction and in contact therewith for not exceeding 20 [56]References Cited seconds.

UNITED STATES PATENTS 3,503,184 3/1970 Knapp et a1. 23/88 X 7 Claims, 1Drawing Figure PAIENIEDSEPZSIQH ceawwm A f lorney ANTI-POLLUTION METHODBACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to environmental control. More particularly, it relates tominimizing escape to the atmosphere of fluoride, particularly hydrogenfluoride, from gas containing same, for example, off gas or effluentfrom electrolytic cells used in the production of aluminum byelectrolysis of alumina.

2. Description of the Prior Art It is known that fluoride, for example,in the form of hydrogen fluoride (HF), may be removed to a considerableextent from gas containing same by adsorption on or chemisorption onparticulate alumina, for example, alumina in a static or fluidized bed.When I refer to fluoride herein, 1 refer to it broadly as encompassingfluorine gas and fluorides, for example, l-IF, that is, fluorine orfluoride (F) in separate form and in combined form, for example, in anycompound containing fluorine or in ionic form. While static andfluidized beds of alumina have proved highly successful in minimizingpollution of the atmosphere by fluoride, there are, nevertheless,problems associated with the use of such. The equipment employed in suchstatic and fluidized bed arrangements, including perforated plates orlike supports, is complicated and expensive to use and maintain.Furthermore, a longer alumina residence time in the bed is generallyemployed than required for maximum alumina sorption efficiency.Therefore, devising an improved and less costly system or method forsorbing fluoride on alumina and one which has a more economical use ofalumina and a shorter time of contact between a particle of alumina andthe stream of fluoride-contaminated gas represents a highly desirableresult.

SUMMARY OF THE INVENTION After extended investigation, I have found thatparticles of alumina can be successfully used for removal of fluoride,particularly hydrogen fluoride, from gas containing same, especiallyeffluent from electrolytic reduction cells, by contacting the aluminawith a stream of the gas moving or flowing concurrently therewith for nolonger than a total contact time of about 60 seconds, preferably lessthan about 20 seconds. Maximum chemisorption of the fluoride on theparticles of alumina, which is believed to be in a sustantiallymonomolecular layer on the surface of the particles, is sometimesaccomplished in as little as 2 seconds or less of total contact timebetween a particle of alumina and the concurrently moving stream of gas.I prefer that the stream of gas and particulate alumina be flowingupwardly in a confined zone. By upwardly I means closer to a verticalthan a horizontal direction, that is, at an upward angle of at leastabout 45. My process is characterized by the fact that during thisconcurrently moving total contact time of not exceeding about 60seconds, the alumina particles are intimately contacted withhighlydilute gas in the dispersed phase.

By highly dilute, I mean that the fluoridecontaminated gas undergoingtreatment according to the invention generally contains less than 50gF/m gas and in many instances as little as about 4 gF/m gas. From myexperience with alumina smelting pots I have found that the off gastreatable according to the invention sometimes contains as small anamount as about 0.1 gF/m when it comes from prebaked pots and as smallan amount as 1 gF/m when it comes from Soderberg-type pots. By myprocess I am able to achieve at least percent removal of fluoride from agas stream, often as high as 99 percent.

The preferred amount of alumina useful according to my invention is from4 to 400 grams of alumina per cubic meter (g/m) of gas being treated,preferably 40-200 g/m Also, according to my invention, the preferredsurface area of the alumina used is at least about 20 m /g, preferably20-200 m /g.

The alumina used as a sorbent according to my invention may convenientlybe of metal grade or smelting grade, that is, of a grade generallyconsidered in the trade as suitable for production of elemental aluminumtherefrom by electrolysis. After use as a sorbent according to myinvention it may conveniently be conducted to smelting pots forelectrolysis therein. The alumina may be sprayed into the stream of gaseither shortly before it enters a vertical tower or column or directlyinto a tower or column of upwardly moving gas close to its entrance atthe bottom of such a tower or column.

While I do not wish to be bound by any particular theory as to why theamount of fluoride sorbed by the alumina is so high or why my process isso highly efficient in removing fluoride, it may be that mixing causedby turbulence created when the velocity of the gas is reduced as itenters the preferred confined zone may be at least partly responsible.

The alumina containing sorbed fluoride may be conveniently separatedfrom the gas stream by one or more conventional-type cyclones, or thelike, the construction and techniques for the use of which are wellknown. After separation from the gas stream, the alumina may be recycledor further used in removing fluoride from the gas stream, if desired,and, as explained hereinabove, also for feed to the electrolytic cell orcells from which the gas stream may emanate.

The temperature in the sorption zone generally ranges from around90-175C.

One of the important discoveries of my invention is that, in removingfluoride by use of concurrently moving gas and particles of adsorbentalumina, intimacy of contact appears to be a controlling factor.According to my process there appears to be an improved intimacy ofcontact between the fluoride and the particles of alumina, thusimproving the efficiency of sorption of fluoride by the alumina.

A particular advantage of my invention is that, if desired, all of thealumina used in removal of fluoride from off-gas from one or more cellsfor electrolytic production of aluminum from alumina may be used asalumina to be electrolyzed in th'eone or more cells, resulting in ahighly economical use of alumina. A further benefit achieved by such anoperation is that when the volume of effluent from the one or more cellsis high, particularly when compared to the fluoride content thereof, andadditional alumina is thus needed to keep the density in the verticalzone high enough, that is, to provide the preferred at least 4 g/m A1 0the additional alumina required may be supplied by recycling to thecontacting zone some of the alumina which accumulates after its firstuse as sorbent.

An additional advantage of the process of my invention is thatcontaminants other than fluoride may be removed from a stream ofpolluted gas by the alumina.

BRIEF DESCRIPTION OF THE DRAWING A further understanding of theinvention will be gained from the drawing which forms a part hereof.

In the drawing, gas containing fluoride, for example, from one or moreelectrolytic cells for production of aluminum from alumina (not shown),enters tower via line 12. Alumina is fed to the tower via line 14. Amotorized fan may be located at 16 for aiding in driving verticallyupward the gas containing the fluoride and the alumina particles throughtower or column 10. Additional gas may be introduced at 16 as desired orneeded to assist in the flow of the gas to be purified and the adsorbentalumina particles vertically through tower 10. Gas containing aluminaparticles with fluoride sorbed thereon is conducted from tower 10 vialine 18 to cyclone or like separator 20 wherein the alumina containingsorbed fluoride is separated from the gas, which exits in purified formvia line 22 to bag filter or other device 24, which may be used, ifdesired, to remove any further fine particulate matter which may stillbe contained therein. The gas may then be emitted to the atmosphere vialine 26 in substantially pure, nonpolluting form. Alumina collected atcyclone or like separating device 20 may be conducted via line 28 to astorage container or the like 30, from which a portion thereof may beintroduced or recycled as needed to tower 10 via line 14 and/or all or aportion thereof conducted to one or more reduction cells (not shown).Additional aluminamay be introduced via line 32. Screw feeders may beemployed both for conducting alumina to the tower and for reintroducingalumina recycled thereto from the cyclone or like separating device 20.

DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE I A 4-m. diameter towersuspension system was installed to handle fumes from 24 prebakedanode-type pots. By use of this suspension system fluoride was removedfrom the pot effluent gas at from 53 to 99 percent efficiency, that is,such that from 53 to 99 percent of the total amount of fluoride in theeffluent stream was removed therefrom. A similar l-m. diameter, 6-m.high suspension system removed 99.9 percent fluoride from the effluentgas from about 30 prebaked anodetype pots, the gas traveling at a rateof about 130 m per minute at about 95C. The velocity in the vertical,approximately l-m. diameter tower was 1.5 m./sec. for the gas movingtogether with the alumina from the bottom toward the top thereof, theheight of the tower being about 6-m. A cyclone was used for collectionof the alumina after sorption of the fluoride thereon. The amount of thealumina introduced near the bottom of the tower was varied from about toabout 25 kg/hr. The maximum loading for this particular tower wascalculated as 4 g/m A1 0 EXAMPLE II A tower approximately l-m. indiameter and 6-m. high is used to remove fluoride from a potline of 31Soderberg-type pots, the gas temperature being about 120C. Effluent gasfrom the 31 pots is fed at about 900 m/min. to the vertical tower orcontactor. The speed of the gas after entrance into the tower is reducedfrom about 9 m/sec, to about 3 m/sec. The contact time for the aluminaand the gas in the tower is about 2 seconds. The alumina-to-gas ratio isvaried from 45-450 g/m of effluent gas from the 31 pots. At g/m thissystem operates at approximately 99 percent efficiency, that is, withabout 99 percent or more of the fluoride being removed from the effluentstream. The alumina containing the fluoride sorbed thereon is removed byusing a cyclone collector.

While the invention has been described in terms of preferredembodiments, the claims appended hereto are intended to encompass allembodiments which fall within the spirit of the invention.

Having thus described my invention and certain embodiments thereof, Iclaim:

1. A process for prevention of pollution of the atmosphere by fluoridecontained in gas released thereto which comprises contacting particulatealumina with a stream of fluoride-containing gas introduced at the lowerportion of a tower containing said alumina so as to flow concurrentlytherewith for a total contact time between a particle of alumina andsaid stream of fluoridecontaining gas of from at least about 2 to notexceeding 60 seconds to an exit point for said alumina andsaid gas inthe upper portion of said tower, thereby sorbing said fluoride on saidalumina, the said gas stream having a fluorine content of from 0.1 to 50grams per cubic meter, and the alumina amounting to about 4-450 gramsper cubic meter of the gas stream.

2. The process of claim 1 wherein said contact time averages from about2 to about 20 seconds.

3. The process of claim 1 wherein the particulate alumina has a surfacearea of from about 20 to about 200 m /g.

4. The process of claim 1 wherein the concurrently flowing particulatealumina and stream of gas are moving in an upward direction.

5. The process of claim 1 wherein at least part of the alumina isseparated and is recycled for further contacting with additionalfluoride-containing gas.

6. A process for removing contaminating fluoride from gas containingsame which comprises introducing particulate alumina and gas containingfluoride in the lower portion of a tower, flowing said alumina and saidgas together from said lower portion to the upper portion of said towerfor a contact time from at least about 2 to not exceeding about 60seconds, conducting said gas and said alumina from said top portion to aseparating zone in which said alumina with fluoride sorbed thereon isseparated from said gas and conducting said alumina with fluoride sorbedthereon and gas substantially free of fluoride separately from saidseparating zone.

7. The process of claim 6 wherein the separating zone comprises acyclone-type separator.

2. The process of claim 1 wherein said contact time averages from about2 to about 20 seconds.
 3. The process of claim 1 wherein the particulatealumina has a surface area of from about 20 to about 200 m2/g.
 4. Theprocess of claim 1 wherein the concurrently flowing particulate aluminaand stream of gas are moving in an upward direction.
 5. The process ofclaim 1 wherein at least part of the alumina is separated and isrecycled for further contacting with additional fluoride-containing gas.6. A process for removing contaminating fluoride from gas containingsame which comprises introducing particulate alumina and gas containingfluoride in the lower portion of a tower, flowing said alumina and saidgas together from said lower portion to the upper portion of said towerfor a contact time from at least about 2 to not exceeding about 60seconds, conducting said gas and said alumina from said top portion to aseparating zone in which said alumina with fluoride sorbed thereon isseparated from said gas and conducting said alumina with fluoride sorbedthereon and gas substantially free of fluoride separately from saidseparating zone.
 7. The process of claim 6 wherein the separating zonecomprises a cyclone-type separator.